1. Field of the Invention
The present invention relates to radio frequency mixers and, more particularly, to balanced mixers.
2. Prior Art
In the context of a communication device, such as a mobile phone, there is a need to combine the RF input signal with a local oscillator signal to generate an intermediate frequency signal for filtering. This function is performed in a mixer and it is this circuit component to which this application is directed.
A singly balanced mixer is shown in FIG. 3, and includes a differential pair of transistors connected to a collector of an RF-amplifier transistor. The local oscillator signal is connected to the bases of the differential switching pair and the RF input is connected to the base of the common emitter amplifier transistor. The RF-amplifier converts input RF-voltage to current at its collector. LO switching transistors alternate the routing of this current to either of the loads.
FIG. 4 shows a double balanced mixer, commonly known as a Gilbert cell mixer. A differential RF transistor pair converts differential RF input voltage to differential current. Two sets of LO switching transistor pairs are used to alternate the routing of the current at the output. The mixer operates as a sign-switcher. The double balanced mixer was designed, among other things, to eliminate the need, in the case of the singly balanced mixer, to cancel out. the local oscillator component from the IF signal. The proliferation of components over the singly balanced mixer, however, creates a sensitive matching challenge. For optimum performance, double balanced mixers require that the component pairs, be matched as close as possible in order to maximize port to port isolation and to reduce undesirable distortion. Conventional integrated double balanced mixer circuits may need compensation for any imbalance by either electrical means or even laser tuning.
It is a purpose of this invention to design a mixer having many of the attributes of the double balanced mixer, while minimizing matching problems. This is accomplished with a reduction in the number of components.
A mixer is provided for mixing radio frequency signals in a communications device. A single pair of matched transistors comprise the core of the mixer. Each of the transistors have their drain port connected to the in phase local oscillator signal(LO) through matching resistance loads and their source port connected to a reverse phase local oscillator signal(xe2x88x92LO) through a resistor acting as a current source. No DC bias is applied between such ports. The gate ports of each of the transistors in the pair receive the differential radio frequency signal. The intermediate frequency output is obtained from the drain ports of the matched transistors. In the positive half cycle of the local oscillator signal the mixer will operate as a differential pair having inverting power gain, while in the negative half cycle of the local oscillator signal, the mixer circuit will operate as two source followers, i.e. two non-inverting amplifiers having current gain. In this manner a mixer is provided with port to port isolation, low noise, and high linearity. This is accomplished with only two sets of matched components.